<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
</body>
</html>
<script>

    var a=1;

    var b;
    b=2;

    var c,d;
    c=3;
    d=4;

    var e=1,f=2;

    // let和var的声明方式一模一样

    let num=123;

    let str;
    str="hello"



</script>